People rely heavily on a wide variety of electrical devices. Almost all of these devices draw power ultimately from a national standard source, usually delivered to the user through a wall outlet or socket. This leads to a challenge for the manufacturer of electrical devices destined for international use: while many electrical devices are sold for use throughout the world, there is no world standard for electrical plug configurations, size, shape, voltage or number of prongs. The wide variety of socket configurations in use worldwide burdens international suppliers of mobile products to varied countries and international travelers who wish to use electrical devices in a portable fashion. Moreover, different areas of the world use different voltage output standards. For example, in Europe and the U.K., the standard is 220 VAC, while in the U.S. the standard is 110 VAC.
Most industrial nations use a standardized alternating current supply with only two leads, a hot side and a neutral side. Some outlets specifically incorporate a separate earth or ground lead while others do not. A problem exists, therefore, with physically accessing an AC current source supplied through any number of outlet configurations and interfacing that current source with the appropriate input connections for a power supply or other electrical device.
The traditional solution for the mechanical prong configuration problem is to provide an adapter which includes a socket to accommodate the prongs of the electrical device integrated with a second set of prongs in a configuration for a local socket. These adapters have some serious flaws. One problem is that the adapters are bulky and at a minimum, cause the prongs of the original device to be extended by at least the length of the additional set of prongs. Since most plug devices are designed to be secured by spring tension and interaction with a wall plug, this can pose a significant mechanical disadvantage. The increased lever arm created by the additional prong length will tend to shift the plug downward, tending to pry the plug out of the wall socket. This will be true even for a light-weight plug.
The lever arm problem is accentuated with devices that are larger than a simple plug. Many power supplies and other electrical devices are designed to be wall-mounted at a wall socket. A typical power supply includes a casing which terminates in a plug designed to plug directly into the wall socket. The casing is often designed to lie against a wall to provide mechanical stability and to maintain the plug prongs in proper contact with the wall socket. If an adapter must be used, the unit loses the stability of resting against the wall and, because even a small amount of weight at the end of a lever arm will create a torque which will tend to pry the prongs out of the wall socket, such a plug adapter is generally unusable for such wall-mounted plug-in devices. Moreover, angling the plug severely can compromise the electrical connection to the point that the plug no longer is in electrical contact with source current. This type of angling may lead to partial separation from the wall socket and may expose the prongs of the plug in such a way that a person or animal might come into contact with live current, thereby causing bodily harm.
Another approach to accommodating multiple physical outlet configurations on a single device is to integrate an interchangeable plug apparatus into the device which allows for easy alteration of the device/outlet compatibility without compromising the device physical characteristics as outlined above. A number of improvements in the art of interchangeable plug design are disclosed in U.S. patent application Ser. Nos. 08/233,125, filed Apr. 26, 1994, 08/414,209, filed Mar. 30, 1995, and 29/044,048, filed Sep. 15, 1995, incorporated herein in their entireties by reference.
Both the adaptor and interchangeable plug approaches to solving the regional differences in outlet configuration fail to address the need to adjust the device electrical circuitry for compatibility with the output voltage associated with each particular source outlet configuration. Some power supply and electrical device manufacturers have addressed this issue by including an input supply adjustment switch on the device or power supply. However, failure to properly adjust such a switch before connecting to the outlet could damage the device, endanger the user and lead to a failure of the electrical circuit connected to the utilized outlet.
Parent application Ser. No. 08/670,247 represents a further refinement in this area. It discloses a system of interchangeable plugs that automatically accommodate the source voltage. This design does require, however, that each plug have an internal wiring configuration appropriate to connect the source voltage with the power supply's transformer coils in order to achieve the correct conversion. Moreover, such interchangeable plugs may be used only with this power supply and conversely, interchangeable plugs designed for other power supplies will not work. Finally, this system requires a three-winding step-down transformer to accommodate the wiring of the interchangeable plugs.
Accordingly, there remains a need for a simplified universal power supply that automatically adjusts to the source voltage and provides a regulated DC output of the desired voltage. There is also a need for such a power supply which can utilize low cost two-winding step down transformers and readily interchangeable plugs that do not require internal wiring. This invention satisfies these and other needs.